Method and apparatus for improving seismic impact signals

ABSTRACT

AN IMPACT BUFFER ZONE OF SPECIAL CONSTRUCTION IS PROVIDED FOR OPERATION IN A SEISMIC SYSTEM WITH A SEISMIC IMPULSE HAMMER THAT IMPACTS THE EARTH TO PROVIDE A SHOCK WAVE. THE BUFFER ZONE, WHEN COUPLED WITH A RIGID PLATE ON THE EARTH, PERFERABLY HAS A SPECIFIED THICKNESS RELATIONSHIP TO PERMIT A SEISMIC WAVE TRAVERSE TIME THROUGH THE BUFFER ZONE GREATER THAN THE TRAVERSE TIME THROUGH THE RIGID PLATE. THE BUFFER ZONE MAY INCLUDE MULTIPLE LAYERS OF ELASTIC MATERIALS SUCH AS WATER, RUBBER, SAND, OIL, OR FOAM PLASTIC. ACCORDINGLY, MEANS AND METHODS ARE DISCLOSED FOR ELASTICALLY ABSORBING A PORTION OF THE IMPACT OF A MOVING MASS GENERATING SEISMIC IMPULSES IN WHICH SUCH A MANNER THAT SEISMIC IMPULSES ARE PRODUCED WITH A RISE TIME OF LESS THAN ONE MILLISECOND.

July 4, 1972 w. H. MAYNE METHOD AND APPARATUS FOR IMPROVING SEISMICIMPACT SIGNALS Original Filed June 16,1965

fzjz/ INVENTOR Wz'ZZad/Az A: 4 4 /1226 uatwpml, M WM' ATTORNEYS UnitedStates Patent 27,418 METHOD AND APPARATUS FOR IMPROVING SEISMIC IMPACTSIGNALS William H. Mayne, San Antonio, Tex., assignor to PettyGeophysical Engineering Company, San Antonio, Tex. Original No.3,367,443, dated Feb. 6, 1968, Ser. No. 464,406, June 16, 1965.Application for reissue Feb. 2, 1970, Ser. No. 8,048

Int. Cl. GOIv 1/14 US. Cl. 181.5 EC 11 Claims Matter enclosed in heavybrackets II] appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT OF THE DISCLOSURE An impact buffer zone of special constructionis provided for operation in a seismic system with a seismic impulsehammer that impacts the earth to provide a shock wave. The buffer zone,when coupled with a rigid plate on the earth, preferably has a specificthickness relationship to permit a seismic wave traverse time throughthe buffer zone greater than the traverse time through the rigid plate.The buffer zone may include multiple layers of elastic materials such aswater, rubber, sand, oil, or foam plastic. Accordingly, means andmethods are disclosed for elastically absorbing a portion of the impactof a moving mass generating seismic impulses in such a manner thatseismic impulses are produced with a rise time of less than onemillisecond.

This invention relates to the provision of seismic signals by means ofimpacting the earth with a moving mass such as a hammer.

Signals for use in seismic surveying or exploring systems have in thepast been developed by impacting the earth with a dropping weight orsome other moving mass. It has been difficult with such impulsegenerators however to generate signals of good uniformity in differentlocations because of variations in the characteristics of the earthssurface. It has been heretofore proposed in my copending applicationSer. No. 314,583, filed Oct. 12, 1963, now Patent No. 3,283,846, forImpulse Seismic Device, that a metal coupling plate be held firmly incontact with compacted earth to improve the etficiency and uniformity ofthe seismic impulses produced by the moving mass. However, under certainconditions the impact device itself may be damaged by excessive stressesafter repetitive use by striking the metal plate at the highest possibleimpact which is attempted to develop seismic impulses of high amplitudeand sharp rise times.

It is therefore a general object of the present invention to provideimproved impact seismic devices which provide increased signal strengthwithout significant increases in stress upon the impact mass.

It is another object of the invention to provide significantimprovements in the amplitude and form of seismic signals which may beused in conventional seismic survey systems.

Another object of the invention is to provide methods and apparatus forimproving seismic impact signals and for providing uniform results.

Thus in accordance with the present invention, impact "ice seismicdevices are provided with an intermediate coupling array between theimpact hammer and the earth. This coupling array serves to improve theuniformity and efiiciency of generating seismic impulses in the earthfor use in a conventional seismic survey system. In general the couplingsystem provides a rigid coupling plate in intimate contact with theearths surface and an elastic buffer material intermediate this plateand the impact hammer to reduce the stresses induced in the hammer.Therefore in accordance with a typical sequence of steps practiced inaccordance with the teachings of this invention, the earths surface iscompacted to provide a bed for a rigid coupling plate in intimatecontact with the earth. The hammer array then is moved into impact withthe buffer material to create seismic shock waves by means of theresultant coupling plate motion.

A further step is to elastically control the impact stresses on themoving mass while maintaining an overall seismic impulse rise time andamplitude not significantly degraded from that obtained with the impactof the hammer on the rigid coupling plate on compacted earth, Thismethod step is carried out by providing an elastic buffer materialbetween a rigid coupling plate and the hammer which receives the impactand thus reduces stresses otherwise induced in the hammer.

The foregoing features of the invention together with other advantagesare defined with particularity in the following specification, takenwith reference to the accompanying drawings, wherein:

FIGURE 1 is a sketch of a mobile impact seismic device which may be usedin accordance with the teachings of this invention;

FIGURE 2 is an elevation view in section of a coupling device forreceiving the impact blow of a seismic hammer in accordance with atypical embodiment of this invention; and

FIGURE 3 is an elevation view, partly in section, of an alternativeembodiment of this invention.

Referring now particularly to the view shown in FIC- URE 1, a seismichammer 10 is provided as carried by a truck 11 or some other devicewhich may'move the impact mechanism 12 from location to location in thefield. At the station 14 shown on the earth 13, the impulse isintroduced by dropping hammer 10 and is used in a seismic survey systemin the making of a seismic recording. Normally the hammer 10 is droppedor driven into an impact with the typical velocity-of 25- to feet persecond at a particular instant of time and thus introduces a seismicshock wave into the earth which is picked up and recorded on asynchronized time basis in a conventional seismic survey system. Theearth is prepacked by the mass 10 as shown by indentation 14 to receivea coupling plate 21 illustrated in place in FIG- URE 2. As shown inFIGURE 3, the coupler plate may be held in place by the weight of thevehicle and the hammer released for spring driven impact.

In operation a buffer plate 22 is employed between the coupling plate 21and the moving mass of the hammer to receive its impact. This may beemployed either as part of the hammer assembly as shown in FIGURE 1, oras part of the coupler plate as shown in FIGURE 2.

For purposes of specific illustration with the butter plate 22embodiment described herein, the effective thickness of the impact massor head 10 might be. for example 5.5 inches. The impact mechanism 12 isnot shown but might be either a triggering device for dropping the Ihammer 10 or in the alternative a'driven hammer as shown in thepreviously mentioned copending application, which is incorporated byreference as part of the background disclosure in this application.

As may be seen from FIGURE 2, the coupling device is inserted in thecompacted earth indentation at station 14 and is used for receiving theimpact of the moving mass. This particular embodiment has a rigidcontact plate 21, which might be a steel plate, in intimate contact withthe earth in the compacted area 14. In general this plate thickness isequal in thickness to the effective head thickness in order to provideproper impact without chatter. An elastic buffer layer 22 is introducedbetween this rigid coupling plate and the hammer impact'surface 23 inorder to elastically absorb the impact of the hammer to avoid excessivestresses in the hammer. While the consistency of this layer may vary, inthis described embodiment the elastic bulfer layer may consist of alayer of sand, which is confined in a suitable container, a layer ofrubber, or a layer of water which'is also suitably conlined in ametallic container, or a combination thereof, as illustrated by thetypical array 25, 26, and 27. Oil, foam, plastic or other materials maybe substituted, as desired.

In general it has been found that no significant deterioration ofseismic survey results will result if rise 1imes of the order of .6- ofa millisecond are not exceeded. This principle is used in connectionwith preparation of :he classic buffer material 22. Thus, if thismaterial has the propagation velocity of 3000 feet per second and is )fsuch critical thickness that the travel time will be :qual to thereflection time through the hammer head, he stress level in the machineor hammer head is reluced by a factor of 6 to l, and under suchcircumstances he soil stress and resultant seismic amplitude is slightyincreased. The rise time of the seismic pulse in this :ircumstance willbe increased to about .3 of a millisecmd which has more than adequatesharpness for seismic :xploration purposes. The stress level can bereduced :ven further at the expense of increasing the rise time If theseismic impulse. Typical figures for stress in the lead and in the soiland the associated rise times are :hown in the following table:

For the value 11 in the previous table, which is the atio of thetraverse time of the seismic impulse wave n the buffer and the headrespectively, the following maerials may be used in the typical elasticbuffer zone 22:

laterial n= 1, in n=8, in

ater (4,800 f.p.s.) 1. 7 5. :ubber (3,000 t.p.s.) 1. 0 3. 1 and, dry(2,500 f.p.s.) 0.9 2. 6

It may be noted from the data given in the foregoing ables that thestresses in the head are considerably re- .uced by providing the elasticbufier zone 22 with a raverse time for the seismic wave at least asgreat as hat through the rigid coupling plate and the impact mass, andyet the seismic impulse in the soil is not eriously deteriorated inamplitude over the range in rhich its rise time is acceptable for 'usein seismic surey systems. It is also to be recognized that with theoregoing apparatus and methods provided in accordnce with this inventionthat uniformity will be mainlined in the introduction of seismicimpulses in various ypes of soil conditions at different surveylocations. Furthermore the invention provides for longer life of seismicimpact apparatus since the stress encountered by the impact mechanism 12or head 10 are significantly reduced. This is particularly significantin connection with use of the power driven seismic head of the naturedescribed in the above identified copending application.

As shown in FIGURE 3, the elastic buifer material may comprise a singlelayer 22' of plastic material producing an effective thickness andpropagation time to satisfy the requirements for elastic stress reliefand impedance match between the hammer 10 and coupling plate 21.

Having therefore provided improved means and methods for generatingseismic impact signals, those novel features believed descriptive of thenature and scope of the present invention are defined with particularityin the following claims.

What is claimed is:

1. In combination, a seismic system comprising an impact mass movablethrough the earths atmosphere to an impact point in contact with theearths surface thereby to create a seismic impulse traveling in theearth, a rigid coupling plate in intimate contact with the earth, atleast one layer of buffer material with elastic properties for absorbingblows disposed in said atmosphere to couple the impact of said mass withsaid rigid coupling plate, wherein said layer of buffer material has aseismic wave traverse time at least as great as the seismic Wavetraverse time through said rigid coupling plate.

2. A combination as defined in claim 1 wherein the seismic wave traversetime through the buffer layer is at least as great as the traverse timethrough the thickness of said impact mass.

3. A combination as defined in claim 1 wherein the impact mass is ofsufiicient thickness and propagation time to equal the elastic stressrelief and impedance between said hammer and said coupling plate [has aneifective thickness equal to the thickness of said coupling plate].

' 4. A combination as defined in claim 1 wherein the seismic wavetraverse times through the rigid coupling plate and the [elfectivethickness of the] impact mass are substantially equal and the seismicwave traverse time through the buffer is at least as great as either.

5. The combination as defined in claim 1 wherein said bufier materialcomprises multiple layers of at least two elastic materials from thegroup of water, rubber, sand, oil and foam plastic.

6. The combination as defined in claim 1 wherein the wave traverse timehas a value between 1 and 5 times the traverse time through thethickness of said rigid coupling plate.

7. A combination as defined in claim 1, wherein the layer of buffermaterial is affixed to said mass and moves with it.

8. In combination, a rigid coupling plate in intimate contact with theearth, at least one layer of intermediary buifer material with a seismicwave traverse time at least as great as said plate and with elasticproperties for absorbing blows, and an impact mass array arranged forimpacting said buffer material.

9. The method of creating seismic impulses comprising the steps of (1)compacting the earths surface, (2) intimately contacting the compactedsurface with a rigid coupling material, (3) impacting said couplingmaterial with a moving mass to create seismic shock waves, and

(4) elastically absorbing a predetermined portion of the impact betweenthe moving mass and the coupling material with an elastic mass having aseismic wave traverse time at least as great as the seismic wavetraverse time through said rigid coupling material while retaining theresulting amplitude and rise time of the seismic impulses to produce arise time of less than one millisecond.

10. The method defined in claim 9 wherein the step of elasticallyabsorbing the impact of said moving mass is accomplished by anintermediary elastic layer having a seismic wave transverse time from[establishing a ratio of] one to five times as great as [between] theseismic wave traverse time through [an intermediary elastic layer and][the traverse time through the head thickness of] the moving mass. 11. Aseismic source for applying energy to the surface of the earthcomprising: a rigid coupling plate; at least one layer of bafiermaterial having elastic properties for absorbing blows; an energy sourceto provide energy to said rigid coupling plate and said at least onelayer of bufier material to generate a seismic impulse; and said buffermaterial having a seismic wave traverse time at least as great as theseismic wave traverse time through said rigid coupling plate.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS MALCOLM F. HUBLER, Primary Examiner

